1. Field of the Invention
This invention relates to a method and apparatus for retrieving operational data from a subterranean borehole. More particularly, this invention relates to a method and apparatus for providing a radio frequency data link between a subterranean device disposed proximate the end of a drill string whereby data related to activity at the end of the drill string can be communicated in real time to the drill rig operator. Although suitable for vertical and non-vertical drilling directions, it is particularly suitable for use in connection with horizontal drilling operations. In addition, it is suitable not only for drilling operations, but also for pullback operations. The invention includes an antenna and corresponding drill head components suitable for inducing the desired data signal into the connected drill string for communication back to the drill operator.
2. Description of Related Art
The technology of drilling gas and oil wells has advanced significantly in recent years. Part of this advancement involves new and improved techniques for drilling non-vertical (i.e. horizontal and other directional) wells. One advantage of horizontal and other directional drilling is that it enables a greater portion of the well bore to be exposed to gas or oil-producing strata, which tend to be disposed more horizontally than vertically. This enables more gas or oil to be produced from the directional well, than from a similar vertical well.
When drilling non-vertical well bores, it is common practice to use downhole sensors to measure the orientation of the well bore. The well bore orientation information gathered during drilling must be transmitted to the surface. Conventional downhole sensors used to measure well bore orientation include a three-axis accelerometer used to measure roll and inclination of the well bore, and a three-axis magnetometer (which functions as an electronic compass) to measure the well bore azimuth. Information on the well bore is conventionally transmitted to the surface of the earth using a wireline, a measurement while drilling (MWD) mud pulser, or an electric dipole.
However, conventional transmission methods and devices have certain disadvantages. Wireline systems, which use a coaxial high strength cable to connect the downhole sensors to the surface, require the use of a wireline truck. Wireline trucks are expensive, both to buy and operate. Also, the wireline must be cut and reconnected to enable the insertion of drill pipe at the surface as the well is drilled down.
MWD methods require changing the downhole fluid dynamics to propagate pressure pulses to the surface. The pressure pulses are used to encode the downhole information. MWD systems are expensive to buy and operate, and do not work well in some formations in which the circulation is lost or poor.
The electric dipole transmission method creates a downhole dipole by electrically isolating a portion of the drill pipe and impressing a voltage across it. This method is relatively simple and inexpensive. However, the technique does not work when there is a moderately conducting formation above the dipole, which shorts the dipole signal. Also, this technique cannot be used inside casings, because the casing shorts out the signal.
Magnetic dipole antenna transmission has been proposed to eliminate the above shortcomings but has yet to be perfected for practical usage.
Yet additional limitations of conventional methods and systems include a lack of range sufficient to deliver real-time data directly to the drilling machine actuating the drill string and an inability to inform the drill operator of conditions along the drill string in real-time to enable timely corrective action to be taken.
A number of systems have short-range capabilities. These typically require that a hand held unit be carried along the drill path so as to be in close proximity to the drill head. The operator carrying the hand-held unit then typically communicates verbally with the operator of the drilling rig. This can lead to delays in stopping operations when undesirable conditions occur. This approach is also unworkable where the horizontal drilling operation is under a railroad, river or busy thoroughfare.
There are also wireless approaches in which data is transmitted from the device in the buried drill string to a movable device on the surface. The data is then automatically transmitted by a second radio link in the surface device to the operator of the drilling rig. This radio-relay approach speeds up the transfer of data to the operator; however, it still requires that the movable relay device be in close proximity to the buried device traveling the drill path.
Other systems have been developed that record conditions at the drill head in an internal memory, which can be extracted after operations are complete. However, such data, although captured, cannot be used in real time to prevent undesirable conditions.
Consequently, there is a need for a method and apparatus for transmitting downhole data to the surface which is relatively simple and inexpensive, which provides a strong signal, which can be used in a wide variety of environments and which provides real-time information to the drilling rig operator.